I think the movie Unstoppable with Denzel Washington associated the word with a runaway train for many people. However, the word originally means "that cannot be stopped or surpassed; unbeatable: an unstoppable ball team." (dictionary.com) Thus, I think it is actually apt word to describe someone who is constantly growing and adapting in order to succeed in their career (and in life). Their forward march can't be stopped because they never stop evolving.